1. Field of the Invention
This invention relates, generally, to apparatus used in the manufacture of components in the compound semiconductor and related industries. More particularly, the invention relates to a crucible for a molecular beam epitaxy (MBE) effusion cell or source. The invention also provides a method and apparatus for the manufacture of the crucible.
2. Background Information
Molecular beam epitaxy is a growth process which involves the deposition of thin films of material onto a substrate in a vacuum by directing molecular or atomic beams onto the substrate. Deposited atoms and molecules migrate to energetically preferred lattice positions on the substrate, which is heated, yielding film growth of high crystalline quality, and optimum thickness uniformity. MBE is widely used in compound semiconductor research and in the semiconductor device fabrication industry, for thin-film deposition of elemental semiconductors, metals and insulating layers.
A principal apparatus utilized in MBE deposition is the thermal effusion cell or source. Thermal effusion cells have a crucible which contains the effusion material, for example gallium, arsenic, or other elements or compounds. The crucible is heated by a resistive filament to heat and effuse the material out of an orifice into an ultra high vacuum growth chamber for deposit on the substrate, which is located in the chamber. Typically, a plurality of cells are mounted, via ports, in the growth chamber. One or more of the cells are actuated and generate a beam which is directed at a predetermined angle toward the substrate which is mounted on a substrate holder. Control of the beam is typically accomplished via shutters and/or valves. In use, various preparatory procedures are performed on the substrate, the cells are powered up, heated and unshuttered, and desired epitaxial deposition is accomplished on the heated, rotating substrate. After growth is completed, the formed wafer is cooled, inspected, and processed for removal from the chamber.
Source crucibles are constructed of an inert material which is stable at high effusion temperatures. A preferred material is pyrolytic boron nitride (PBN). The crucibles are typically formed by a chemical vapor deposition (CVD) process utilizing a forming mandrel in a vacuum chamber. In the past, various crucible designs and configurations have been used in MBE. However, these prior art crucibles have significant limitations. The primary problems associated with existing crucibles are: (1) low capacity, (2) lack of uniformity, (3) oval defect production, (4) short term flux transients, and (5) long term flux transients.
Capacity relates to the ability of the crucible to hold an amount of material necessary for a particular MBE process. Greater capacity permits construction of larger and/or a greater number of devices per load of source material. Desired capacity has been achieved in some designs by utilizing a straight-wall, cylindrical configuration. However, crucibles having a cylindrical configuration throughout tend to provide poor depositional uniformity because the beam emitted from the zero draft cylindrical orifice is too tightly limited.
Uniformity relates primarily to the uniformity of the thickness of the layers deposited over the target substrate area via the material emitted from the orifice of the crucible. Uniformity may also be compositional. Uniformity has been achieved in some designs by utilizing a conically configured crucible body with a positive draft. However, crucibles having a conic configuration throughout have limited capacity, exhibit depletion effects, and are prone to flux transients.
Oval defects are morphological defects present on the formed semiconductor device. Source related oval defects are thought to be caused by spitting from the material melt at the crucible base which occurs when droplets of condensed material form at the crucible orifice and then roll back into the melt. Material condenses at the orifice due to a reduced temperature in the orifice region. Oval defect production has been reduced in some designs by heating the orifice or lip of the crucible to prevent material condensation. Such designs are commonly referred to as "hot lip" devices. A problem with some hot lip source designs is that they produce a hydrodynamically unstable flux, they tend to produce undesirable levels of impurities, and they often exhibit depletion effects.
Short term or shutter-related flux transients are changes in the effusion rate over time due to the activation of the source shutter. Long term flux transients are changes in effusion rate over time due to decreases in the surface area of the melt. Flux transients are particularly a problem in crucible designs having a conic configuration throughout.
Short and long term flux transients have been reduced in a design manufactured by applicants' assignee which utilizes a dual filament crucible heating system along with a straight-wall, cylindrical crucible body combined with a conic insert at the orifice end. In the dual filament system, one filament heats the base of the crucible and another filament, which is controlled independently, heats the lip of the crucible. This yields a "hot lip" system which reduces oval defect production and also minimizes hydrodynamic instability and rapid depletion effect typically experienced in hot lip crucible designs. Further, the large crucible volume provided by the straight wall crucible in combination with the insert, forms a thermal baffle between the melt and the shutter further improving hydrodynamic stability. Although this design represents an advance over other prior art crucibles, it appears to have a shortcoming; namely the lip heating filament is not believed to be optimally disposed in close proximity to the conic insert due to the presence of the outer wall of the cylindrical crucible body.
Despite the need in the art for an effusion cell crucible design which overcomes the disadvantages, shortcomings and limitations of the prior art, none insofar as is known has been developed.
Accordingly, it is an object of the present invention to provide a unibody, monolithic, negative draft crucible for a MBE effusion cell. It is a further object of this invention to provide a crucible which maximizes capacity, uniformity and long term flux stability, and minimizes oval defects, depletion effects, and short term shutter-related flux transients. It is a further object of this invention to provide a method and apparatus for making a unibody containment structure, such as a crucible formed of PBN, having a negative draft, via chemical vapor deposition. Finally, it is an object of the present invention to provide a unibody, one-piece crucible, and a method and apparatus for its manufacture.